Let me put it this way. If Rivers isn't the QB in 2016, who is?
Based on how beat up a 33yo Rivers was by the end of the season last year I can't say with any certainty that he will make it through 2015. As of today I think SD has one NFL quality starting lineman. Based on the protection the team has put in front of him the past several years my hopes aren't as high as they probably should be they are going to significantly change that. He'll turn 35 during the 2016 season and I have no idea what his health will look like at that point.
Just to be clear, it seems you are advocating to let it ride with his contract, see how he plays in 2015, and then sign him if warranted. Is that right?
Do you expect the team will keep him beyond 2015? If yes, doesn't it then make sense to extend him and gain more cap space to use
while they still have him? Odds are that when Rivers no longer starts for the Chargers, they will go through an extended period of lousy QB play before they find another good QB.
Rivers is among the very best Chargers of all time, in the group with Alworth, LT, Winslow, Fouts, and Seau, in no particular order. I think an argument can be made that he is the best Chargers player of all time. I think there is virtually no chance the Chargers will not keep him beyond next season, unless he decides to retire, which seems unlikely. I do not expect to ever see Rivers play for another team.
Anyone who goes along with that premise should be 100% in favor of the team extending his contract to provide immediate cap relief that can be used to improve the team in the short term.